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Safety wire or lockwire is a type of positive locking device that prevents fasteners from loosening due to vibration and other forces. The presence of safety wiring also serves to indicate that the fasteners have been properly tightened.
Safety wire is available in a variety of gauges and materials, depending on the application. In aircraft and racing applications, stainless steel wire is used, most commonly in 0.032 in (0.81 mm) diameter. Typically, the wire is threaded through a hole drilled into a fastener or part, then twisted and anchored to a second fastener or part, then twisted again.
There are a few techniques for different applications. Safety wire is not used to keep a fastener or nut in place. It is made out of aluminum and is meant to break, thus allowing for its true purpose, which is to give indication to a mechanic/inspector to visually see if torque is broken and the nut or bolt is loosening. The wire itself maintains tension and remains in place by being twisted around itself and attached to the fastener to be secured on one end and an anchor point (which could be another fastener) on the other end. Since safety wire is made of a malleable alloy, it retains its shape after being bent, rather than springing back to its original shape. This property allows it to remain locked around an object, such as when it is passed through a small hole on a fastener, looped back upon itself and then twisted. The same process is then repeated around the anchor point, which could be another fastener. Since it remains twisted instead of unraveling, it acts as a fixed loop and will not back out without considerable force (greater than the stresses which it is intended to counter) being applied.
Mousing (pronounced mouzing) is the application of safety wire, called mousing wire in this use, to secure a threaded clevis pin to a shackle. This is done by passing a couple of turns of mousing wire through the hole provided for this purpose in the unthreaded end of the clevis pin and around the body of the shackle's hoop. Alternatively, some threaded shackles are provided with a hole through the threaded end of the pin beyond where it emerges from the threaded hole. A cotter pin or a couple of loops of mousing wire through this hole serves the same purpose and secures the shackle in a closed position. Nylon zip ties are also commonly used in applications where the shackle must be secured, but easy removal is required.
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Safety wire installation is a fairly simple task, but needs to be done properly to ensure proper security for the fastener in question. First, a length of wire is cut off of a spool. The length of the piece of wire to be used is determined by the distance from the location of the fastener to be secured to another fixed location which will act as an anchoring point for the safety wire. The wire should be cut to a length that is slightly longer than twice that distance. The next step is to take the removable fastener, such as a bolt, drill a hole through a part of it that will be accessible even when it is fastened (such as the head of the bolt), fasten it in the desired location, and then thread one end of the safety wire through the hole and pull it back upon itself, such that the two ends of the wire are now equal in length and form a loop through the fastener. The wire is then grabbed a short distance before the loose ends (with enough wire left so that it can be secured around the anchor point) with the tool of choice and pulled taut. Using safety wire pliers or the twisting tool of choice (simple pliers can be used effectively, if not efficiently), the wire is twisted to the optimal number of twists per inch. The wire is then released from the tool, with the two loose ends remaining adjacent to each other. One of those ends is then threaded through a hole on the anchor point, and pulled through and brought together with the other loose end, such that a loop is formed around the anchor object and the wire between the anchor object and the original fastener is twisted and barely taut. With the two loose ends now on the opposite side of the anchor point (with one end going through the anchor and one going around it), the twisting process is repeated on the loose ends. When finished, the loose end is cut off with a pair of diagonal cutters or wire cutters (many safety wire pliers have a wire cutting feature), such that enough twists are left to prevent the wire from unraveling.
Safety wire is not reusable so when it is removed it is cut.
Proper techniques 
When using the most common gauge of safety wire, which is .032", guidance for installation can be found in several publicly available sources. When using .041 or .040", optimal performance is achieved by twisting the wire 5 to 7 times per inch. On the smallest 0.020" wire, 8 to 10 twists per inch is optimal.
Equally important to the size of wire and number of twists, is the direction the safety wire is run from the fastener to the anchor point. The safety wire should be threaded through the object fastener such that it creates tension in the opposite direction of the fastener's removal. For example, if a standard automotive bolt in the U.S. is being secured, then the safety wire when installed should put tension on the bolt in a clockwise direction, since that is the direction that the bolt turns to tighten.
When drilling a fastener, the choice of where to drill it depends on the type of fastener and to what it will be wired. The alternative to drilling holes in fasteners is to use safety wire tabs (see Safety Wire Tabs section below), or to purchase pre-drilled fasteners.
Safety wire 
Safety wire is commonly .020, .025, .032 or .041" in diameter, but .021 and .051" are also available. It is usually made of stainless steel, but is also available in monel and inconel alloys for high temperature applications and copper for break-away applications. For consumer applications, it is typically sold in 1 pound spools enclosed in a small cardboard or plastic canister.
Safety wire pliers 
Safety wire installation pliers (SWIPES) come in three common sizes: 6, 9, or 12 inch(also called 7, 10, 13 inch, respectively, for manufacturers who choose to include the knob at the end of the pliers, which is used to actuate the twisting of the pliers, in the measurement). Safety wire pliers work as follows: The technician grabs the two loose ends of the wire to be twisted in the end of the pliers; he then locks the handles of the pliers closed, which secures the loosed wire ends in the jaws; he then grabs the knob at the end the pliers, releases the main body of the pliers, and pulls back on the knob which causes the pliers to twist. The wire then twists. When the wire has been twisted sufficiently, the technician holds the pliers, releases the knob, and squeezes the handles of the pliers to release the locking mechanism, which loosens the jaws, thereby releasing the twisted wires. He then proceeds to the next fastener in the series or uses the pliers' built-in wire cutters to trim off the excess wires. The final step is the push down the sharp end of the secured safety wire to eliminate any hazard from the sharp edge left from the excess being cut off.
In addition to built-in wire cutters, some safety wire pliers feature an angled or diagonal tip to their jaws which allows easier grabbing of wire. The angled tip is also useful for pushing down the sharp clipped end of the secured safety wire. Some models have a feature that catches the excess wire that is cut off. This consists of a pair of rubber pillows located in the depression aft of the cutting blades.
Safety wire pliers also vary in the direction of their rotation when the twisting knob is pulled, with some pliers rotating clockwise, some rotating counterclockwise (often called Right-Hand Twist and Left-Hand or Reversing Twist), and yet others have a selector function to allow the user to twist the wire in either direction without switching tools. The pliers can also be purchased with the option of the twisting knob returning to its position automatically or manually.
Safety wire twisters 
A safety wire twister is a simple tool that allows the user to grip the two loose ends of a piece of safety wire and then, while holding the main barrel of the tool, turn the end that is not gripping the wire (which is bent to create a simple cranking mechanism) in order to twist the safety wire.
There is another type of basic safety wire twister which is similar to a standard screwdriver, except that the tip has a small grasping mechanism to hold the ends of the wire while the technician turns the handle to twist the wire. The advantage to this tool is its long and thin design, which can access hard-to-reach areas where one's hands or pliers do not fit. It is commonly referred to as a "pignose" due to its snout-like appearance.
Safety wire tabs 
Safety wire tabs are washers that are used to secure fasteners by transferring the force of the safety wire to the head of the fastener to be secured. They are installed just like any other washer, after which the sides of the tab are bent up to make contact with the sides of the head of the fastener. One side of the tab is longer than the other with a small hole at the top, through which safety wire is threaded. Once the Safety Wire is properly installed, the sides of the tab transfer the force of the safety wire to the fastener, as though the fastener itself had been drilled and had the safety wire run though it.
The advantage of safety wire tabs is that the fastener to be secured does not need to be drilled, which can be advantageous for fasteners that should not or cannot be drilled because of size or damage concerns. They can also be useful when a fastener needs to be replaced, the replacement is not already drilled, and circumstances do not afford the time or tools to properly prepare the replacement fastener. The disadvantages are that it adds extra distance between the head of the fastener and the surface to which it is to be secured, and it is not as secure as securing the wire directly to the fastener itself as the tab could be a point of failure if it somehow unbends or the hole breaks (which is more likely than the hole in a drilled fastener failing due to the thinness and malleability of the material from which it is made).
Pre-drilled fasteners 
For certain applications where safety wiring is common, fasteners come pre-drilled with holes to accept safety wire. When wiring something that did not come with pre-drilled fasteners stock, however, the more cost effective way (as opposed to replacing all stock fasteners with pre-drilled ones of the same type) is often to drill the stock fasteners.
Drilling jigs 
Because the use of twisted safety wire to secure fasteners requires the fasteners to be drilled, tool makers offer jigs to help technicians drill the fasteners to be secured. Although pre-drilled fasteners can be obtained, most fasteners to be secured start out never having been intended to be secured (e.g., a production motorcycle which was built for the street but which has been converted into a race-bike). Such fasteners need to be drilled. Drilling them is often difficult as, due to their small size and irregular shape, securing them properly and applying a drill effectively can be trying. As a result, technicians often break drill bits or damage the fastener when the bit slides off position. A useful tool is a drill press, because it allows the technician to apply the force of the drill bit directly to the fastener being drilled and eliminates lateral movement; but even with a press the fastener needs to be secured to prevent it from sliding out from beneath the bit. Even though drill presses ease the process, a press isn't always available, such as at a race event; and even with a press, the problem of securing the fastener still exists. To solve those problems, jigs are available which are designed to securely hold the fastener and provide a guide-channel for a drill bit (with either a hand drill or a press) so that the technician can easily and directly apply force from the drill to the fastener without having it slip off or breaking the bit.
Proprietary methods 
Safety cable is an alternative to safety wire. Safety cable is defined as a group of strands right-hand helically twisted without a core. This eliminates the need for twisting during installation, as is required with safety wire. Several companies manufacture safety cable, and it is becoming an industry standard due to easier control of critical inputs and reduced installation time. Installation and quality requirements of safety cable are governed by SAE AS4536.
One safety cable manufacturer Daniels Manufacturing Corporation of Orlando, Florida. Daniels manufactures a product called Safe-T-Cable, which they advertise as, "The complete fastener retention system."
The Daniels system works by providing pre-cut lengths of safety wire that have a large cap on one end. The cable is threaded through a hole on the fastener to be secured, which is large enough to accommodate the wire but too small for the cap on the other end of the cable to pass through it. After the other end of the wire is passed through the anchor point, the technician takes an extra end cap and the special tool that is available from Daniels and crimps the cap on to that end of the wire. Again, the end cap is too large to pass through the hole that the wire passed through, and thus the cable is secured.
Advantages and disadvantages 
Although many systems purport to be more efficient than installing traditional safety wire, an advantageous by-product of the twisting method of installing safety wire is that it leaves a highly visible and easily inspectable indication that the fasteners in question are in fact properly secured.
In addition, safety wire twisting is a standard, non-proprietary technique, and tools and materials can be easily found, cheaply purchased, and mixed with other brands while still working properly (provided of course that all components are used properly and with the proper types of complimentary components and tools, if not brands).
The primary disadvantage of traditional safety wire is the time it requires to install properly when securing fasteners, although technicians who use it often can implement it fairly quickly. It also leaves behind waste products when ends are clipped off or when it is cut off of secured fasteners that need to be removed during maintenance, resulting in sharp metal bits that can easily damage soft materials or injure skin. However, the amount of waste product is relatively small, it is non-toxic, and the hazard can be mitigated altogether if technicians properly dispose of any waste product. When clipping off ends, the ends can go flying off which makes their recovery difficult and can cause injury to anyone in the immediate vicinity, such as the technician or an assistant; however, this can also be easily mitigated by using extra care or by using safety wire pliers that have a special insert that is designed to catch clipped off ends. Another disadvantage is that since the manual skill required to implement traditional safety wire is easily learned, the techniques required to maximize the retentive force of safety wire (e.g., in which direction the retentive force should be exerted, the direction of twist, proper angles for securing multiple fasteners, proper twists per inch, which type of wire to use, etc.) are often ignored by non-formally trained technicians (e.g., hobbyists) who use safety wire for their projects.
There are also other systems of fastener retention that do not rely on safety wire at all, such as lock washers, thread-locking fluid and cotter pins, all of which accomplish the same objective as safety wire, which is to prevent vibration or other forces from causing the undesired loosening of a mechanical fastener.
- FAA AC 43.13-1B, Section 7
- SAE AS 567
- SAE AS4536
- Daniels Manufacturing Corporation